There is no treatment and no cure, but his father Alex has given up his job to dedicate himself to finding one. He hopes to achieve this by funding research through his charity Harrison's Fund.

A campaign for the charity features a picture of Alex and Harrison and carries the slogan: "I wish my son had cancer".

Explaining the advert, which appeared in the London Evening Standard, Alex said: "For many people the notion of even wishing your child had cancer would be tough to understand, how could I wish a terrible disease on my own child?

"But the brutal truth is that there is a treatment in most cases. With duchenne there is no treatment, no cure, it's 100 per cent fatal and at the moment one day he will be dead."

Alex, from Cobham, Surrey, who has another son William, four, with his wife Donna, 37, said when Harrison was first diagnosed in 2011, 'the world turned upside down'.

He said: "Coming to terms with a child that has a life-limiting condition, it's not what we had mapped out for ourselves. We were also worried that we could have two sons with duchenne.

"I'm a very determined guy and I can't accept being told, 'Look after your son, give him the best life you can, he is going to die.' I couldn't accept that solution."

As a result, he decided to give up his job in company branding to start a charity named after Harrison in a bid to find a practical solution for him and other sufferers.

Duchenne muscular dystrophy, which is caused by a genetic mutation in the dystrophin gene located on the X chromosome, affects around one in 3,500 boys in the UK.